Referring to FIG. 1, a conventional control apparatus for a charging generator will be described. In FIG. 1, reference numeral 10 designates a generator, 1 a three-phase winding of a stator , 2 a field coil, 3 a rectifier, 31, 32 and 33 first, second, and third rectifier terminals, respectively, 4 a storage battery, 5 an indicator lamp, and 6 a key switch. Further, reference numeral 7 designates a control apparatus, 71 a switching element such as a switching transistor for turning on/off the field current in the field coil 2, 72 a transistor for controlling the switching transistor 71, 73 a zener diode used for detecting the output voltage of the generator 10, 74a and 75b, first and second resistors, respectively, constituting a first voltage dividing circuit, 74b and 75b third and fourth resistors, respectively, constituting a second voltage dividing circuit, and 76a and 76b diodes respectively connected in the first and second voltage dividing circuits and constituting an OR gate.
The operation of the apparatus of FIG. 1 will be described hereunder. Upon turning on the key switch 6, a current flows from the storage battery 4 to the switching transistor 71 through the key switch 6, the indicator lamp 5, and the field coil 2 so that the indicator lamp 5 is lit and the field coil 2 is excited. Then, the engine is started and the generator 10 is driven to generate electric power. A voltage appears at each of the first and second output terminals 31 and 32 so that a charging current flows from the first output terminal 31 into the storage battery 4 to both start the charging of the battery and turn off the indicator lamp 5. The indicator lamp 5 is turned off because the same voltages are produced at the output terminals 31 and 32, and therefore the potentials at the two terminals of the indicator lamp 5 are equal to each other.
When the rate of rotation of the generator is further increased and the division voltage at a first detection point a rises above a predetermined value, the zener diode 73 is energized, the control transistor 72 is made conductive, and the switching transistor 71 is turned off. Accordingly, the current in the field coil decreases to lower the voltage generated by the generator 10. Then, the switching transistor 71 is turned on again to increase the output voltage of the generator. In this manner, the output voltage of the generator 10 is held at a predetermined value.
The division voltage at a second detection point b (a second set value) is set to a higher value than the division voltage at the first detection point a (a first set value). Thus, if disconnection occurs in the first detection circuit for some reason such as a connector coming loose, the control operation of the apparatus is performed using the second set value, thereby preventing overcharging of the storage battery 4.
In the AC generator of this type, ripples are included in the rectified current; the larger the output current, the higher the ripples become. Further, there is a disadvantage that the zener diode 73 may be energized in spite of an insufficient value of the output voltage of the generator 10 because the zener diode 73 is energized by the peak value of the output of the generator while the storage battery 4 is charged with the mean value of the output of the generator 10.